This invention relates to electronic filters, and more particularly to tunable filters of the type used in radio-frequency (RF) systems such as RF communications and test equipment, for example, in RF front-end sections of receivers and transmitters.
The continuous rapid development of wireless communication technologies has led to significant growth of RF and microwave devices and systems for both civilian and military applications. This is particularly true for mobile applications with increased demands for high efficiency, small volume, low weight and low cost. Furthermore, there is a defining trend to combine multiple wireless communication functionalities into a single frequency-agile platform for multi-band and multi-standard operation with a lower overall system cost. While digital technologies and the associated signal processing have enabled the implementation of frequency agility at base-band, the design and implementation of the RF front-end components remain a significant challenge for mobile systems. Tunable filters suitable for band selection and image rejection constitute typical examples.
Most of today's practical tunable filters are based on the ferromagnetic tunability Yttrium-Iron-Garnet (YIG) resonators. YIG based tunable filters exhibit very wide tuning range over multiple octaves and very high quality factors (Q) of 10,000 at 10 GHz. Nevertheless, the large volume and high power consumption (0.75-3 W) of YIG based tunable filters hinder their integration into mobile communication systems. Many alternative approaches have been proposed to make miniaturized tunable RF/Microwave filters. These approaches are predominantly based on planar transmission line resonators loaded with solid-state varactors, ferroelectric-tuned varactors and MEMS varactors, switches and switched capacitors, as described, for example, in the following papers:    A. R. Brown et al., “A Varactor-Tuned RF Filter,” IEEE Trans. Microwave Theory & Tech., Vol. 48, No. 7, pp. 1157-1160, July 2000;    F. A. G. Miranda et al., “Design And Development Of Ferroelectric Tunable Microwave Components For Ku- and K-Band Satellite Communication Systems,” IEEE Trans. Microwave Theory & Tech., Vol. 48, No. 7, pp. 1181-1189, July 2000;    A. Tombak et al., “Voltage-Controlled RF Filters Employing Thin-Film Bariumstrontium-Titanate Tunable Capacitors,” IEEE Trans. Microwave Theory & Tech., Vol. 51, No. 2, pp. 462-467, February 2003;    J. Nath et al., “An Electronically Tunable Microstrip Bandpass Filter Using Thin-Film Barium-Strontium-Titanate (BST) Varactors,” IEEE Trans. Microwave Theory & Tech., Vol. 53, No. 9, pp. 2707-2712, September 2005;    D. Peroulis et al., “MEMS Devices For High Isolation Switching And Tunable Filtering,” 2000 IEEE MTT-S Int. Microwave Symp. Dig., Vol. 2, No., pp. 1217-1220 vol. 2, 2000;    A. Abbaspour-Tamijani et al., “Miniature And Tunable Filters Using MEMS Capacitors,” IEEE Trans. Microwave Theory & Tech., Vol. 51, No. 7, pp. 1878-1885, July 2003; and    A. Pothier et al., “Low-Loss 2-Bit Tunable Bandpass Filters Using MEMS DC Contact Switches,” IEEE Trans. Microwave Theory & Tech., Vol. 53, No. 1, pp. 354-360, January 2005.
While achieving significant progresses in terms of miniaturization and tuning, planar resonator based tunable filters have relatively low quality factor (Q) of less than 400-500. This is due to either the low-Q lumped elements, as in the case of solid-state varactors, and/or the low-Q of the resonator itself. Inherently high-Q resonators loaded with tuners that result in a graceful Q degradation while tuning are need for high-Q tunable filters. Several approaches have attempted to accomplish this by employing dielectric resonators, cavity resonators and high temperature superconductivity (HTS) resonators. However, their limited tuning range and the requirement for cryogenic cooling (in the case of HTS tunable resonators) makes them unsuitable for mobile systems in the near future.
Evanescent-mode waveguide filters have recently attracted interest for realizing low-loss, highly-selective tunable filters for reconfigurable RF front-ends. Reference is had to the following papers:    T. A. Schwarz et al., “A Micromachined Evanescent Mode Resonator,” 1999 European Microwave Conference Dig., Vol. 2, pp. 403-406, October 1999;    L. P. B. Katehi, “Tunable Evanescent Mode Filters,” DARPA Project Report, December 2000; and    X. Gong et al., “Precision Fabrication Techniques and Analysis on High-Q Evanescent-Mode Resonators and Filters of Different Geometries,” IEEE Trans. Microwave Theory & Tech., Vol. 52, No. 11, pp. 2557-2566, November 2004.
Compared to half-wave cavity resonators, evanescent mode resonators offer several advantages including substantially smaller volume and weight, larger spurious-free region and feasibility for monolithic integration while maintaining a very high-Q. It is well known that waveguides below cut off can be used to create microwave filters by introducing obstacles inside the guide, as described by G. F. Craven et al. in “The Design of Evanescent Mode Waveguide Bandpass Filters for a Prescribed Insertion Loss Characteristic,” IEEE Trans. Microwave Theory & Tech., Vol. 19, No. 3, pp. 295-308, March 1971, and by R. V. Snyder in “New Application of Evanescent Mode Wave-Guide to Filter Design,” IEEE Trans. Microwave Theory & Tech., Vol. 25, No. 12, pp. 1013-1021, December 1977. This concept finds wide applications in the form of evanescent mode filters, ridge waveguide filters and combline filters. See, e.g., R. Levy et al., “Transitional Combline/Evanescent-Mode Microwave Filters,” IEEE Trans. Microwave Theory & Tech., Vol. 45, No. 12, pp. 2094-2099, December 1997; A. Kirilenko et al., “Evanescent-Mode Ridged Waveguide Bandpass Filters With Improved Performance,” IEEE Trans. Microwave Theory & Tech., Vol. 50, No. 5, pp. 1324-1327, May 2002; and R. V. Snyder et al., “Bandstop Filters Using Dielectric Loaded Evanescent Mode Resonators,” 2006 IEEE MTT-S Int. Microwave Symp. Dig., Vol. 2, No., pp. 599-602 Vol. 2, 6-11 Jun. 2004.
The simplest and most practical type of waveguide obstacle is a conductive re-entrant post, which represents an effective shunt capacitance and is usually realized by a tuning screw. By changing this capacitance, i.e., the gap between the post and waveguide wall (FIG. 1), the center frequency of the filter can be changed. It has been proposed to create tunable evanescent-mode filters using metal thin films as waveguide walls and externally attached piezoelectric actuators for frequency tuning, as described by S. M. Hou et al. in “A High-Q Widely Tunable Gigahertz Electromagnetic Cavity Resonator,” J. Microelectromech. Syst., Vol. 15, No. 6, pp. 1540-1545, December 2006, and by H. Joshi et al. in “Highly Loaded Evanescent Cavities for Widely Tunable High-Q Filters,” 2007 IEEE MTT-S Int. Microwave Symp. Dig., pp. 2133-2136, June 2007. These techniques offer excellent RF performance but suffer from issues such as large overall volume, hysteresis, instability, and/or high power consumption. One alternative is electrostatic actuation, offering advantages as discussed herein, but there remains a need for improvements in actuation techniques for tunable evanescent-mode filters.
Another shortcoming of current filters in front-end receivers is that they have very limited tuning. Generally multiple filters are needed in a multi-band environment. Also, the filter bandwidth tends to vary linearly with frequency in evanescent-mode filters due to the coupling iris, which can be quite significant over tuning ranges of more than an octave.